1. Field of the Invention
The present invention relates to a method of evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas which is disposed in the exhaust passage of an internal combustion engine.
2. Description of the Related Art
Generally, catalytic converters such as three-way catalytic converters are disposed in the exhaust passages of internal combustion engines for purifying gas components including HC (hydrocarbon), NOx (nitrogen oxides), etc. contained in the exhaust gas emitted from the internal combustion engines.
Each of the gas components is purified by the catalytic converter at a substantially maximum purification rate when the air-fuel ratio of the exhaust gas upstream of the catalytic converter is of a certain value or a value close thereto. If the actual air-fuel ratio is shifted from the certain value to a fuel-lean value or a fuel-rich value, then the purification rate of the gas component tends to be lowered. The air-fuel ratio of the exhaust gas upstream of the catalytic converter specifically is recognized from the concentration of oxygen in the exhaust gas that enters the catalytic converter and represents the air-fuel ratio of an air-fuel mixture which is burned by the internal combustion engine to produce the exhaust gas. The air-fuel ratio will hereinafter referred to as the air-fuel ratio of the internal combustion engine.
The catalytic converters of the above type are gradually deteriorated, with their purifying capability lowered, due to continued use over time. In order to keep the exhaust gas purified adequately by a catalytic converter, the catalytic converter needs to be replaced with a new one when it is deteriorated to a certain extent. For properly replacing the deteriorated catalytic converter with a new one, it is necessary to rely on a technology to evaluate a deteriorated state of the catalytic converter appropriately.
In recent years, there have been developed exhaust gas sensors, specifically HC sensors and NOx sensors, capable of detecting relatively accurately the concentrations of various gas components, including HC, NOx, etc., to be purified by catalytic converters. Some of these exhaust gas sensors generally have their output levels increasing substantially linearly as the concentrations of the detected gas components increase. The output levels of other exhaust gas sensors decrease as the concentrations of the detected gas components increase. The output characteristics of the exhaust gas sensors of the former type will hereinafter be referred to as positive characteristics, and the output characteristics of the exhaust gas sensors of the latter type will hereinafter be referred to as negative characteristics.
Since the above exhaust gas sensors are capable of detecting the concentrations of gas components that are actually purified by a catalytic converter, i.e., the concentrations of gas components which are directly affected by the deteriorated state of the catalytic converter, the exhaust gas sensors can effectively be used to recognize the deteriorated state of the catalytic converter.
For example, two exhaust gas sensors are disposed respectively upstream and downstream of the catalytic converter, and while the internal combustion engine is in operation, the ratio of output levels of those exhaust gas sensors is determined to directly recognize the purification rate of the gas component detected by the exhaust gas sensors and purified by the catalytic converter. The purification rate decreases as the deterioration of the catalytic converter goes on. Therefore, if the purification rate of the gas component is recognized as described above, then it is possible to appropriately determine the deteriorated state of the catalytic converter.
However, due to the effect of a response delay of the catalytic converter, it is actually difficult to accurately recognize the purification rate of the gas component detected by the exhaust gas sensors and purified by the catalytic converter. Attempts to increase the accuracy of the purification rate that can be recognized by canceling the effect of the response delay of the catalytic converter are liable to be complex.
For the above reasons, there has been a demand for a new technology capable of appropriately determining the deteriorated state of a catalytic converter according to a relatively simple process using exhaust sensors such as HC sensors and NOx sensors.
It is therefore an object of the present invention to provide a method of appropriately evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas, according to a relatively simple process using an exhaust sensor which detects a gas component such as HC, NOx, or the like to be purified by the catalytic converter.
To achieve the above object, there is provided a method of evaluating a deteriorated state of a catalytic converter which is disposed in an exhaust passage of an internal combustion engine for purifying an exhaust gas flowing through the exhaust passage, using data of the output of an exhaust gas sensor which is disposed downstream of the catalytic converter for detecting the concentration of a particular component of the exhaust gas purified by the catalytic converter while the internal combustion engine is in operation, comprising the steps of identifying the values of parameters contained in a nonlinear function of a predetermined type for expressing the correlating characteristics of the concentration of the particular component detected by the exhaust gas sensor with respect to an air-fuel ratio of the exhaust gas upstream of the catalytic converter, using data of the air-fuel ratio of the exhaust gas upstream of the catalytic converter and the data of the output of the exhaust gas sensor, and evaluating the deteriorated state of the catalytic converter based on the identified values of the parameters.
The quadratic function has an independent variable represented by the air-fuel ratio of the exhaust gas upstream of the catalytic converter, e.g. the output of a sensor for detecting the air-fuel ratio, the air-fuel ratio recognized from the output of the sensor, or a target value of the air-fuel ratio, and a dependent variable represented by the output of the exhaust gas sensor or the concentration of the particular component recognized from the output of the exhaust gas sensor. The nonlinear function may be a function of higher degree, e.g., a quadratic function, a cubic function, or the like. The parameters specifically comprise parameters which determine the configuration of the graph of the nonlinear function. For example, if the nonlinear function is a function of higher degree, e.g., a quadratic function, a cubic function, or the like, then the parameters comprise coefficients of terms of the different degrees and a constant term thereof.
The air-fuel ratio of the exhaust gas upstream of the catalytic converter specifically comprises an air-fuel ratio recognized from the concentration of oxygen contained in the exhaust gas. The air-fuel ratio may be referred to simply as an air-fuel ratio of the internal combustion engine.
The particular component detected by the exhaust gas sensor may comprise HC (hydrocarbon), NOx (nitrogen oxide), or the like. Thus, the exhaust gas sensor may comprise an HC sensor, an NOx sensor, or the like.
In the above identifying step, the values of the parameters of the nonlinear function are identified using data of the air-fuel ratio of the internal combustion engine, i.e., the data of the air-fuel ratio of the exhaust gas upstream of the catalytic converter, and the data of the output of the exhaust gas sensor, i.e., the data of the detected value of the concentration of the particular component, thus determining the nonlinear function which approximately expresses, with an equation, the correlating characteristics of the concentration of the particular component detected by the exhaust gas sensor with respect to the air-fuel ratio of the internal combustion engine, i.e., determining the configuration of the graph of the nonlinear function.
The concentration of the particular component detected by the exhaust gas sensor, i.e., the concentration of the particular component of the exhaust gas that has been purified by the catalytic converter, generally tends to have a minimum value with respect to a change in the air-fuel ratio of the internal combustion engine. Such correlating characteristics can appropriately be expressed by a nonlinear function of a suitable type, e.g., a quadratic function.
When the values of the parameters of the nonlinear function are identified in the above identifying step, the configuration of the graph of the nonlinear function determined by the identified values of the parameters tends to exhibit a certain characteristic change. Specifically, the identified values of the parameters of the nonlinear function reflect the deteriorated state of the catalytic converter. According to the present invention, the evaluating steps evaluates the deteriorated state of the catalytic converter based on the values of the parameters of the nonlinear function which are identified in the above identifying step.
Since the identified values of the parameters of the nonlinear function are determined by a statistical process, the identified values of the parameters of the nonlinear function are less susceptible to response delay characteristics of the catalytic converter, and properly reflect the deteriorated state of the catalytic converter. The values of the parameters of the nonlinear function can be determined according to a known process such as a method of least squares or the like. Therefore, the deteriorated state of the catalytic converter can appropriately be evaluated according to a simple process.
The data of the air-fuel ratio of the internal combustion engine, i.e., the data of the air-fuel ratio up-stream of the catalytic converter, used to identify the values of the parameters of the nonlinear function in the identifying step, may be data representing the value of the air-fuel ratio itself, but should preferably be data representing the difference between the air-fuel ratio of the internal combustion engine and a predetermined reference value.
The values of the parameters of the nonlinear function which are identified using the data representing the difference between the air-fuel ratio of the internal combustion engine and the predetermined reference value are of increased accuracy.
If the air-fuel ratio of the internal combustion engine is controlled at a predetermined target air-fuel ratio, then it is possible to use the target air-fuel ratio as the air-fuel ratio of the internal combustion engine for identifying the values of the parameters of the nonlinear function. Preferably, however, an air-fuel ratio sensor is disposed upstream of the catalytic converter for detecting the air-fuel ratio of the internal combustion engine, and the step of identifying the values of parameters comprises the step of using the air-fuel ratio detected by the air-fuel ratio sensor as the air-fuel ratio of the internal combustion engine for identifying the values of the parameters of the nonlinear function.
The values of the parameters of the nonlinear function which are identified using the air-fuel ratio detected by the air-fuel ratio sensor, i.e., the actual value of the air-fuel ratio of the internal combustion engine (the air-fuel ratio of the exhaust gas upstream of the catalytic converter), are of increased accuracy.
With the air-fuel ratio sensor, the method further comprises the steps of determining the value of an air-fuel ratio which minimizes the concentration of the particular component represented by a function value of the nonlinear function with the values of the parameters thereof being identified, using the identified values of the parameters of the nonlinear function, and obtaining the determined value of the air-fuel ratio as a target air-fuel ratio of the exhaust gas upstream of the catalytic converter, and manipulating the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine according to a feedback control process to converge the air-fuel ratio detected by the air-fuel ratio sensor to the target air-fuel ratio.
The nonlinear function determined by the values of the parameters which are thus identified approximately expresses the correlating characteristics, i.e., the correlating characteristics of the concentration of the particular component detected by the exhaust gas sensor with respect to the air-fuel ratio of the internal combustion engine. Therefore, when the target air-fuel ratio of the internal combustion engine, i.e., the target air-fuel ratio of the exhaust gas upstream of the catalytic converter, is determined as described above, the target air-fuel ratio basically becomes a target air-fuel ratio which minimizes the concentration of the particular component detected by the exhaust gas sensor, i.e., the concentration of the particular component which has been purified by the catalytic converter. Consequently, when the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine is manipulated according to a feedback control process to converge the air-fuel ratio detected by the air-fuel ratio sensor to the target air-fuel ratio, the air-fuel ratio of the internal combustion engine is controlled at an air-fuel ratio state which maximizes the purification rate of the particular component by the catalytic converter. Thus, it is possible to well purify the particular component with the catalytic converter, i.e., to purify the particular component at a maximum purification rate, while evaluating the deteriorated state of the catalytic converter.
The air-fuel ratio of the internal combustion engine should preferably be manipulated according to the feedback control process by a recursive-type feedback control means such as an adaptive controller. The recursive-type feedback control means determines a new feedback controlled quantity according to a given recursive formula containing a predetermined number of time-series data, prior to the present time, of the feedback controlled quantity of the air-fuel ratio, e.g., a corrective quantity for the amount of supplied fuel.
The parameters of the nonlinear function may be identified after data representing the air-fuel ratio of the internal combustion engine (the output of the air-fuel sensor or the like) and data of the output of the exhaust gas sensor are collected and accumulated. However, the step of identifying the values of parameters comprises the step of sequentially identifying the values of the parameters of the nonlinear function according to a sequential identifying algorithm.
Since the sequential identifying algorithm is used, a memory capacity required to execute the sequential identifying algorithm may be small. Because the parameters of the nonlinear function are sequentially updated on a real-time basis, it is possible to obtain the identified values of the parameters of the nonlinear function depending on behavioral states of the catalytic converter and the internal combustion engine from instant to instant. As a result, the deteriorated state of the catalytic converter can be evaluated highly reliably based on the identified values of the parameters of the nonlinear function.
The sequential identifying algorithm may comprise any one of the algorithms of a sequential method of least squares, a sequential method of weighted least squares, a fixed gain method, a degressive gain method, etc. According to these algorithms, new values of the parameters are determined, i.e., the values of the parameters are updated, in order to minimize an error or difference between the value of the output of the exhaust gas sensor determined according to the nonlinear function using the present identified values of the parameters and the actual value of the output of the exhaust gas sensor.
In the present invention using the sequential identifying algorithm, the step of identifying the values of parameters preferably comprises the step of identifying the value of at least one of the parameters while limiting the value of the at least one of the parameters to a value satisfying a predetermined condition.
Specifically, the parameters of the nonlinear function include such a parameter that if the value thereof is erroneously identified as a value falling out of a certain desired range due to disturbances or the like, the actual configuration of the graph of the correlating characteristics becomes largely different from the configuration of the graph of the nonlinear function. According to the present invention, when at least one of the parameters of the nonlinear function is to be identified, the value of the parameter is limited to a value which satisfies a certain condition. In this manner, the value of the parameter is made reliable, increasing the reliability of an evaluating parameter that is determined from the value of the parameter.
For example, if the quadratic function comprises a quadratic function, then the step of identifying the values of parameters should preferably comprise the steps of regarding a coefficient of a term of maximum degree of the quadratic function as the at least one of the parameters which is to be limited, and identifying the value of the coefficient while limiting the value of the coefficient in order for the concentration of the particular component represented by the function value of the quadratic function to have a minimum value.
Specifically, the function value of the quadratic function may have a minimum value or a maximum depending on whether the value of the coefficient of the term of maximum degree thereof is positive or negative. Therefore, if the polarity, positive or negative, of the identified value of the coefficient is inappropriate, then the concentration of the particular component represented by the function value of the quadratic function as the nonlinear function has a maximum value, rather than a minimum value which the concentration is to have. Therefore, the identified value of the coefficient is limited such that the concentration of the particular component represented by the function value of the quadratic function has a minimum value. Specifically, the identified value of the coefficient is limited to either a positive or negative value. In this manner, the reliability of the identified value of the parameter and hence the evaluating parameter is increased.
When the concentration of the particular component represented by the function value of the quadratic function has a minimum value, the function value of the nonlinear function may have either a minimum value or a maximum value. Specifically, if the dependent variable of the nonlinear function is represented by the concentration of the particular component recognized from the output of the exhaust gas sensor, then the function value of the nonlinear function has a minimum value at the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function. If the dependent variable of the nonlinear function is represented by output of the exhaust gas sensor, then the function value of the nonlinear function has a minimum value at the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function insofar as the output of the exhaust gas sensor has positive characteristics with respect to the concentration of the particular component, i.e., such characteristics that the output increases as the concentration increases. If the dependent variable of the nonlinear function is represented by output of the exhaust gas sensor, then the concentration of the particular component represented by the function value of the nonlinear function decreases as the function value increases insofar as the output of the exhaust gas sensor has negative characteristics with respect to the concentration of the particular component, i.e., such characteristics that the output decreases as the concentration increases. In this case, therefore, the function value of the nonlinear function has a maximum value at the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function.
When the value (identified value) of the coefficient of the term of maximum degree of the quadratic function is limited, if the dependent variable of the quadratic function as the nonlinear function is represented by the concentration of the particular component recognized from the output of the exhaust gas sensor, or the output of the exhaust gas sensor having the positive characteristics, then the value of the coefficient of the term of maximum degree of the quadratic function is limited to a positive value, and if the dependent variable of the quadratic function is represented by the output of the exhaust gas sensor having the negative characteristics, then the value of the coefficient of the term of maximum degree of the quadratic function is limited to a negative value.
Based on the identified values of the parameters of the nonlinear function, the deteriorated state of the catalytic converter is specifically limited as follows:
The step of evaluating the deteriorated state of the catalytic converter comprises the steps of determining the value of an evaluating parameter defined, using the parameters of the nonlinear function, as representing a change depending on the progress of deterioration of the catalytic converter, from the identified values of the parameters of the nonlinear function, and evaluating the deteriorated state of the catalytic converter based on the value of the evaluating parameter.
When the evaluating parameter determined by the identified values of the parameters of the nonlinear function is introduced, the value of the parameter exhibits a change depending on the progress of deterioration of the catalytic converter, i.e., increases or decreases as the deterioration of the catalytic converter progresses. Therefore, the deteriorated state of the catalytic converter can be evaluated by comparing the value of the evaluating parameter with a predetermined value.
More specifically, the graph of the nonlinear function with the values of the parameters being identified tends to be shifted in its entirety into the leaner air-fuel ratio range as the deterioration of the catalytic converter progresses.
Therefore, one example of the evaluating parameter may be an evaluating parameter (hereinafter referred to as xe2x80x9cfirst evaluating parameterxe2x80x9d) which changes as the graph of the nonlinear function with the values of the parameters being identified in the identifying step is shifted into the leaner air-fuel ratio range as the deterioration of the catalytic converter progresses.
More specifically, the first evaluating parameter may comprise the value of an air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function.
Based on the fact that the concentration of the particular component detected by the exhaust gas sensor generally has a minimum value when the air-fuel ratio of the internal combustion engine changes, the concentration of the particular component represented by the function value of the nonlinear function also has a minimum value. When the graph of the nonlinear function is shifted into the leaner air-fuel ratio range as the deterioration of the catalytic converter progresses, the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function changes to a value in the leaner air-fuel ratio range.
Thus, such value of the air-fuel ratio can be employed as the first evaluating parameter. The deteriorated state of the catalytic converter can appropriately be evaluated based on the first evaluating parameter.
The graph of the nonlinear function with the values of the parameters being identified tends to be shifted in its entirety in a direction to increase the concentration of the particular component represented by the function value of the nonlinear function as the deterioration of the catalytic converter processes. This is because as the deterioration of the catalytic converter processes, the purification rate of the particular component is lowered, increasing the concentration of the particular component detected by the exhaust gas sensor.
Another example of the evaluating parameter may be an evaluating parameter (hereinafter referred to as xe2x80x9csecond evaluating parameterxe2x80x9d) which changes as the graph of the nonlinear function with the values of the parameters being identified in the identifying step is shifted in the direction to increase the concentration of the particular component represented by the function value of the nonlinear function as the deterioration of the catalytic converter progresses.
The second evaluating parameter may comprise an extremal value (maximum or minimum value) of the function value of the nonlinear function which corresponds to a minimum value of the concentration of the particular component represented by the function value of the nonlinear function.
Specifically, as the deterioration of the catalytic converter progresses, the concentration of the particular component represented by the function value of the nonlinear function increases. Therefore, the extremal value of the function value of the nonlinear function which corresponds to the minimum value of the concentration changes similarly. If the extremal value of the nonlinear function is a minimum value, then the extremal value increases as the deterioration of the catalytic converter progresses. If the extremal value of the nonlinear function is a maximum value, then the extremal value decreases as the deterioration of the catalytic converter progresses.
Thus, the extremal value of the nonlinear function can be employed as the second evaluating parameter. The deteriorated state of the catalytic converter can appropriately be evaluated based on the second evaluating parameter.
According to the nonlinear function with the values of the parameters thereof being identified, as the deterioration of the catalytic converter progresses, the gradient of the graph of the nonlinear function increases in the air-fuel ratio range leaner or richer than, or basically in the air-fuel ratio ranges on both sides of, the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function. This is because as the deterioration of the catalytic converter progresses, the air-fuel ratio range of the internal combustion engine which is capable of well purifying the particular component is reduced.
Still another example of the evaluating parameter may be an evaluating parameter (hereinafter referred to as xe2x80x9cthird evaluating parameterxe2x80x9d) which changes as the gradient of the graph of the nonlinear function in the air-fuel ratio range leaner or richer than the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function with the values of the parameters thereof being identified in the identifying step, increases as the deterioration of the catalytic converter progresses.
Specifically, the third evaluating parameter may be the function value of the nonlinear function at the value of the air-fuel ratio changed a predetermined quantity into the leaner or richer air-fuel ratio range from the value of the air-fuel ratio which minimizes the concentration of the particular component represented by the function value of the nonlinear function.
When the gradient of the graph of the nonlinear function increases as the deterioration of the catalytic converter progresses, the function value of the nonlinear function increases or decreases at the value of the air-fuel ratio changed a predetermined quantity into the leaner or richer air-fuel ratio range from the value of the air-fuel ratio which minimizes the concentration of the particular component. In this case, if the extremal value of the nonlinear function corresponding to the minimum value of the concentration of the particular component is a minimum value, then the function value increases as the deterioration of the catalytic converter progresses. If the extremal value of the nonlinear function is a maximum value, then the function value decreases as the deterioration of the catalytic converter progresses.
Thus, the function value of the nonlinear function can be employed as the third evaluating parameter. The deteriorated state of the catalytic converter can appropriately be evaluated based on the third evaluating parameter.
The deteriorated state of the catalytic converter may be evaluated based on only one of the first, second, and third evaluating parameters. Preferably, however, the evaluating parameter determined in the evaluating step comprises a plurality of evaluating parameters, i.e., the first, second, and third evaluating parameters are determined. The step of evaluating the deteriorated state of the catalytic converter should preferably comprise the step of combining evaluations of the deteriorated state of the catalytic converter based on the plurality of evaluating parameters to evaluate the deteriorated state of the catalytic converter.
In this manner, the deteriorated state of the catalytic converter can be evaluated with increased reliability.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.